Broadband ISDN (B-ISDN) networks are expected to support a wide variety of services, including voice, data, video and multimedia, on a single network platform with standardized interfaces. One current area of intense research interest is whether B-ISDN protocols can be used to satisfy the requirements of gigabit/second computer communications. If this is shown to be feasible, gigabit applications may become widespread since it is anticipated that B-ISDN networks will be widely deployed.
The B-ISDN Protocol Reference Model provides a framework for B-ISDN protocols in the form of a layered communication architecture. The lowest layer is the Physical Layer, which is based on the Synchronous Optical Network (SONET)/Synchronous Digital Hierarchy (SDH). SONET/SDH defines a standard set of optical interfaces for transport of data over a network. The next higher layer is the Asynchronous Transfer Mode (ATM) Layer, which provides for the transfer of fixed-size data units (cells) between source and destination with an agreed-upon grade of service. Both the Physical and ATM Layers are service-independent.
The third B-ISDN layer is the ATM Adaptation Layer (AAL) which is responsible for adapting different higher-layer services to the ATM layer. The AAL is service-dependent and as a result several different types of AALs have been defined to handle services with different characteristics. These characteristics include both constant (CBR) and variable bit rates (VBRs), connectionless and connection-oriented, and timing requirements between source and destination. Currently, five AAL types have been defined.
From the point of view of constructing user-network interfaces, it may be desirable to implement several or all of the AAL types within a single interface. Furthermore, it may also be desirable to support a flexible interface to allow for future changes in AAL formats as well as new AALs to be developed. Hence, this would allow a great deal of flexibility in terms of the services that the interface could provide.